I’ve mentioned laryngeal and tracheal anatomy a few times on Tet Zoo (see the links at the very bottom for more). Well, time to look at it again. It’s (relatively) little known that a long list of mammal species possess an assortment of ‘pouches’, pocket-like structures and pneumatic sacs and spaces within their throats, skulls, chests, and sometimes on their palates. Some of these are air-filled, epithelium-lined structures that originate as outgrowths of the throat or windpipe, and are hence known as diverticula (singular: diverticulum) [image below shows Siamang Symphalangus syndactylus (from wikipedia) with inflated throat pouch, and a diagrammatic great ape showing the extent of the laryngeal pouches (based on a diagram in Meldrum (2006)].

As someone interested in pneumaticity, the mechanics and anatomy of respiration and vocalising, and in sheer weirdness, I’ve been planning for ages to write some articles that summarise our knowledge of these structures. It’s a fairly big subject, so I ended up writing quite a lot: this article is part 1 of a series of several.
Before looking at the structures of interest, a brief (and very basic) anatomical primer (for this and the following articles you’ll need to be familiar with the following terms… skip the next few paragraphs if you already know the anatomy).

A brief anatomical primer

The pharynx is the name for the space between the back of the mouth (and nasal cavity) and the most anterior part of the trachea: the trachea is the tube (held open by cartilaginous rings) that connects the pharynx to the bronchi and eventually the lungs. The upper part of the trachea is occupied by a specialised vocal compartment (and some associated glands), the larynx. The larynx is complicated, consisting of various different cartilaginous components and membranes, and contains the vocal folds (or vocal cords). The vocal folds and the immediate surrounding space are known as the glottis [some of these structures are shown in the adjacent sagittal section, ultimately from Gray’s Anatomy but borrowed here from wikipedia].

Within the pharynx, the epiglottis is a projecting cartilaginous process located close to the base of the tongue, and normally kept in contact with the soft palate or velum (humans are unusual [though probably not unique] in possessing an epiglottis that is located some distance down the throat, and a short velum: this combination means that there is no contact between the two structures). In between the tongue and epiglottis is a shallow recess termed the epiglottic valecula: this is the part of the mouth where chewed food is formed into a bolus for swallowing. The nasal cavity, larynx and trachea can all be referred to collectively as the upper respiratory tract.

Finally, note that I won’t be talking about the various sinuses that are located within the nasal region, forehead, middle ear and temporal region of mammals (some mammals do possess the same combination of sinuses that we do, others do not). I might, however, mention some of the sinuses in passing.

Ok, time to move on…

Among the huge variety of air-filled sacs and spaces present in mammals, perhaps the ones that are of most interest to people are those seen in other hominids [male orangutan shown here, from here. The pendulous pouch on the chest is mostly occupied by laryngeal pouches]. Orangutans, gorillas and chimpanzees possess paired, inflatable sacs – termed laryngeal pouches (and actually representing hypertrophied versions of the laryngeal ventricles*) – that run along the sides of the neck and upper part of the chest [image below – from Fitch (2000) – compares the respiratory tract anatomy of (a) orangutan, (b) chimp and (c) human. The tongue is in red, the larynx in yellow and the air sacs in blue**. The non-human MRIs were provided by Sugio Hayama and Kiyoshi Honda]. The larger gibbons (like the Siamong shown at top] also possess these laryngeal pouches, as do some guenons, baboons, macaques and howler monkeys (Hewitt et al. 2002).

* Laryngeal ventricles (sometimes called the ventricles of Morgagni) are bilateral folds, located on the inside walls (close to the glottis) of the larynx. They are present in all primates. Some primates possess additional ventricles on either the dorsal (some lemurs and spider monkeys) or ventral (some lemurs and various New World and Old World monkeys) inside surfaces of the larynx.

** Air sacs across Tetrapoda are consistently shown in blue, even though there’s no proper convention on this sort of thing.

Studies have shown that the pouches of primates are inflated during exhalation (yes, exhalation: not inhalation), and that the air they contain is removed during inhalation. They can be surprisingly large, with those of orangutans extending to 6000 cubic cm when inflated (and thus being the biggest of any primate). Primate air sacs can be prone to bacterial infection: numerous species kept in captivity have been treated for air sac disorders and there’s a large veterinary literature on the subject, particularly in apes. Believe it or don’t, the commonest disease affecting air sacs is known as airsacculitis (the same name is used for air sac infection in birds).

As is the case with most structures present in living animals, the function of these diverticula is tremendously under-studied, but it’s widely thought that they act as resonating chambers used during vocalising. Other proposed acoustic roles include that they function as impedance matchers, or that they suppress resonance. An acoustic function is supported by the fact that the calls of a monkey (a De Brazza’s monkey Cercopithecus neglectus) were shown in a 1971 experiment to be lower in intensity after the pouches were punctured. And – in monkeys at least – the pouches are larger in males than females, and it’s males who make the louder calls.

However, other possible functions have been proposed: perhaps they help prevent hyperventilation during long bouts of calling, or perhaps they somehow strengthen the ribcage during climbing (yeah, don’t ask me how that one’s supposed to work). In another experiment, air sacs were surgically removed from a macaque (a Rhesus macaque Macaca mulatta), and its calls after the operation were reportedly no different than they were before. These observations perhaps suggest that the air sacs of primates have different roles in different species.

A question that’s been asked many times concerns their distribution within hominids. If non-human hominids have them, did humans possess them ancestrally? It’s generally agreed that they were lost some time between the divergence of the human lineage from the chimpanzee one. The good news is that their presence can be inferred from the shape of the hyoid bone. In chimps and some other primates with laryngeal pouches, the anterior part of the hyoid is enlarged, anteriorly convex and forms a hollow bulla (these are, so far as we know, the only cases in mammals of tracheal diverticula invading the skeleton). It’s well known that this hyoid bulla is over-the-top developed in howler monkeys (Alouatta) where it’s enormous, particularly in males [howler monkey – labelled as A. seniculus, but used in the old, inclusive sense of the name – with hypertrophied hyoid bulla shown here; photographed at the Cambridge University Museum of Zoology].

One australopithecine specimen – the juvenile A. afarensis from Dikika in Ethiopia (sometimes called ‘Selam': shown here, from wikipedia) – definitely has a bulla, so australopithecines do seem to have had air sacs (Alemseged et al. 2006)*. Other fossil hyoids show that, like modern humans, at least some fossil members of our genus (H. neanderthalensis and H. heidelbergensis) lacked bullae, and thus probably lacked air sacs as we do. However, we can’t be absolutely sure about that because Pongo – which very definitely does have air sacs – has a simple hyoid without the bulla shape.

* Because the Dikika specimen is a juvenile, it’s always possible that hyoid shape changed during ontogeny.

Nevertheless, for now it seems fairly sensible to assume that air sacs were present in early members of the human lineage, but lost somewhere between Australopithecus and the younger members of Homo. Why were air sacs lost in the human lineage? Several proposals have been made. Perhaps they were lost because humans switched to quieter forms of communication, or perhaps finer control over breathing patterns meant that humans no longer needed assistance from air sacs (Hewitt et al. 2002). However…

When humans have air sacs

We modern humans are not ordinarily supposed to have enlarged tracheal diverticula. In some people – as many as 15-30% of adults according to some studies – either one of the laryngeal ventricles is abnormally long, extending superiorly above the level of the thyroid cartilage at the top of the larynx (note that ‘abnormally long’ still means c. 5-15 mm). These enlarged ventricles are termed internal laryngocoeles: they’re internal because they don’t extend beyond the limits of the larynx. Because enlarged laryngeal ventricles are, as we’ve seen above, normal for other hominid species, it’s been suggested that big ventricles in humans might be atavistic.

Some long ventricles in some people reach – and even pierce – the thyrohyoid membrane right at the top of the larynx (normally only pierced by the internal laryngeal nerve and superior laryngeal artery), and thus become external laryngocoeles. Sometimes the laryngocoeles have a dumbbell shape and consist of both internal and external components; the ‘waisted’ portion in the middle being that part that squeezes through the thyrohyoid membrane (Giovanniello et al. 1970). The x-rays below show an external laryngocoele in a patient; from Giovanniello et al. (1970).

Enlarged laryngeal ventricles in humans might result from congenital deformity, but it’s more likely that they result from excessive build up of pressure in the glottis. Glass-blowers, people who play saxophones and other wind and brass instruments, and people who suffer from chronic coughing are therefore mostly the ones who develop these structures [diagram below showing internal and external laryngocoeles, from Giovanniello et al. (1970). When both internal and external laryngocoeles are present, the term ‘mixed laryngocoele’ is used… but this is redundant and unnecessary, given that external laryngocoeles only occur when internal ones are already present].

Due to their interference with the vocal cords, internal laryngocoeles produce chronic hoarseness, and can also create the impression of throat blockage and shortness of breath. External laryngocoeles, however, may not ordinarily cause any obvious negative effects other than creating an unsightly soft bulge on the side of the neck. This bulge can change size and shape as the owner performs the Valsalva manoeuvre (the process where you forcibly exhale without releasing the air from the mouth or nose). However, there are several cases of external laryngocoeles causing death following acute obstruction of the upper respiratory tract.

The really neat thing about laryngocoeles is that they demonstrate how diverticula can herniate surrounding structures and then sit in new positions, sometimes close to the skin and visible externally. If an animal that possessed these was provided with some sort of selective advantage (say, they assisted flotation, in enhancing the acoustics of calls, or were somehow attractive to mates or intimidating to rivals), you can imagine that they might become a fixed feature. Maybe this is how the inflatable nasal balloons of certain seals and other animals evolved.

Moving away from laryngocoeles for a moment, people have learnt that they can create pouch-like structures in their throats by keeping lead discs inside their pyriform sinuses, thereby enlarging the sinuses over time (the pyriform sinuses are yet another set of paired pockets, this time positioned on either side of the opening to the larynx). These resulting pouches – termed ganahs – were said by Guthrie (in Anon. 1927) to be used by Indian criminals “as a safe hiding place for coins or precious stones”.

Sasquatch and the Aquatic Ape Hypothesis

One last thing to say about primate air sacs. You might find it interesting that diverticula have been mentioned in a few ‘non-standard hypotheses’ on hominid diversity and evolution. As we’ll see in a later article, inflatable air sacs are present in some marine mammals, and probably play a buoyancy role in some pinnipeds. Unsurprisingly, proponents of the aquatic ape hypothesis (= AAH) have therefore suggested that air sacs might be something to do with the amphibious stage they imagine for hominids. As with other pieces of data used to support the AAH, the logic here doesn’t quite follow given that air sacs are biggest in primates that have nothing to do with water, nor is there any reason for thinking that the primates that have them use them on the (mostly rare) occasions when they wade or swim.

The possible presence of hypertrophied laryngeal pouches has also been discussed a few times in the literature on sasquatch (Krantz 1999, Meldrum 2006). If sasquatch is a non-human ape, the presence of enlarged air sacs would be expected. Furthermore, it’s been suggested that the loud vocalisations inferred by some to be created by sasquatches indicate the presence of giant, orangutan-like air sacs in this animal. Grover Krantz even considered the idea that the large, human-like breasts of ‘Patty’ – the subject of the 1967 Patterson-Gimlin film – might be partially inflated air sacs, but noted that this was more unlikely than the idea that they really are breasts (Krantz 1999) [Patty and her famous breasts shown in the adjacent image] [UPDATE: my comments inspired this article over on Cryptomundo].

And on that note… time to leave! Next: elephants with pouches.

If you’re interested in tracheae and their role in respiration, vocalising and such, or on any of the associated structures in the neck, check out…

Comments

As someone very much interested in the evolution of vocal behavior, I’ve been hoping just such an article would happen on here for as long as I’ve been reading/lurking – here’s to this becoming the longest series yet!

If air sacks can form in humans due to high pressure in the respiratory tract, is it possible that their size in apes is partly due to routine production of high pressure in the respiratory tract also, perhaps from calls? The shape and form would obviously still be guided by selection, but I wonder if a mute orangutan might have hypotrophied air sacks.

Thanks for comments. Mike (comment 3): impedance matching (in acoustics) refers to what I sort of regard as the amplification of transmission created by a structure (the impedance matcher) when waves travelling through one space (or medium) enter another. The usual example involves the middle ear, where waves travelling across the large, low impedance tympanum are transferred to the small, high impedance oval window of the cochlea: because the energy from the tympanum is focused on a smaller, fluid-filled structure, impedance matching means that the energy arrives at the oval window with something like 1.4 times more force than it did at the tympanum. For laryngeal air sacs, the proposal for an impedance matching function is that a high rate of vibration inside the sacs is needed in order to force the acoustic energy out through the vocal folds… or something like that.

I’m in the Pacific NW and don’t believe in no sasquatch. My friend the true believer won’t hear anything I have to say that’s less than 100% supportive of the possibility it’s real, which is a fair bit more obnoxious than simply agreeing to disagree like a sane person. The complexity of the human brain seems like a curse more often than a boon.

The only hypothesis I’ve heard that I’d entertain is that ‘squatch leaves no trace evidence because he’s a creature of the spirit world. We have no good reason to believe in the spirit world either, but at least it has a built-in excuse for being beyond the kenning of mortals.

As far as the orang goes – it may be creating a tool for some purpose of its own. One orang I heard of (I believe at Chicago) got hold of a piece of wire and kept it secreted in its mouth until it was unobserved (except for a security camera), then used it to pick the lock on its enclosure.

@Alan
If you can trace this orangutan story to the souce, I will be interested. There are lots of true stories of zoo apes breaking out using very amazing means. I know of one orangutan in Chester who used sticks to shortcut the elcteric fence.

@LeeB- OMG! That’s so cool that I must die. X_X
Of the San Diego orangutan, my gf says, “That’s what you get for giving him a human name.” With a name like that, he was probably looking for a corner office and a nice subdivision to live in.

I am inclined to agree that hominidae chauvinists are wrong and orangutans are more intelligent than chimpanzees.

That may be the case, but one can’t draw that conclusion from incidents like this; there are other plausible explanations. For example, if orangutans indeed are generally better than chimpanzees at escaping from zoo cages (an unnatural environment for either species), perhaps that’s mainly because they tend to be more patient and/or because they have longer attention spans than chimpanzees? Alternatively, they could be better at escaping for some morphological reason (e.g., better climbing adaptations, or greater manual dexterity).

Or, to put it somewhat differently: If you locked up, for example, Socrates, Macchiavelli, Mozart, Einstein, and Houdini in zoo cages, who of them do you think would most quickly and/or most easily escape? And would that person be the one who’s generally the smartest of the lot?

I read recent opinion of some researchers that orangutans in very complex trials like eg. using a tool to make another tool indeed are better than chimpanzees.

These funny stories just add to this. BTW, orangutans are indeed very patient, which was considered to be disadvantage, because absolute time to solve puzzles was longer for orangutans than chimps. They also have famously strong hands, being able to unscrew huge, industrial bolts using fingers.

BTW, I think typical tests for animal intelligence are too restrictive and simply unable to measure very complex skills. I never seen the test which measures these orangutan advanced observation skills and creativity.

How to make a Patty (IMO) – take one very large male Homo sapiens (possibly with some form of acromegaly or pituitary giantism – think Andre the Giant or similar pro-wrestlers), add fake breasts (because a female that big and robust would be much further out of the “plausibly-human” zone, given general hominid sexual dimorphism), spray all over with glue, then cover in hair clippings. The apparent hairless rectangualar strip across the eyes and nose can be explained by goggles worn for eye protection at the glue stage.

Use this method instead of the Hollywood-fursuit method for a reconstruction, and you’d have a much more accurate assessment of whether “Patty” is a genuine non-sapiens hominid or not.

(Of course, if Sasquatch is a Gigantopithecus or close relative thereof, as some have suggested, then the orangutan is its closest living relative, and it’s a true pongid (pongine?), and therefore parsimony would suggest it probably has similar air sacs. If it’s a real animal, though, i find that theory unlikely due to the extremely humanlike leg and foot anatomy.)

A fascinating (and very long) article on orangutan escapes can be read here. It’s kind of sad that some individuals spend just about all of their time devising new ways of escaping from captivity – maybe this indicates that, you know, they shouldn’t be in captivity?

Most frogs live in the Amazon rainforest and never come closer to the ground than maybe 20 meters. Pipids, permanently aquatic frogs without a dead layer on top of their skin, retain full croaking abilities, IIRC. Orang-utans are not much less of canopy specialists than gibbons are. And so on. “Precise parallels”? You’re counting the hits and ignoring the thousands of misses.

1. African dwarf frog is an erroneous model for an anuran that has ‘lost air sacs’, since it belongs to a clade (Pipoidea) that has lacked vocal sacs throughout its history. They have not lost vocal sacs; they never had them. As for correlations between diving and ‘loss of air sacs’, what about all the cetaceans and pinnipeds that have inflatable air sacs?
2 and 3. Air sacs cannot be seen as special to climbing taxa that spend time near the water, nor are they lost in canopy dwellers. The big gibbons with large sacs (Siamangs etc.) have no special association with water, nor do the majority of monkeys with large laryngeal sacs, and I don’t think the correlation you note for orangutans is valid.

The sacs of anurans and primates are nothing alike anyway and don’t reveal special convergence (other than that they are big, inflatable sacs used in vocalising).

@35 DN: I await your next post on air sacs before commenting, but I suspect that the pipids did have some sort of (rudimentary) tongue and throat air sac in the past which was completely lost due to strong selection for improved suction feeding in a aquatic phase. Do only female pipids click, while males buzz underwater?

Bob Heironimus was the guy in the Bigfoot suit. He was on the bigfoot hunting trip when the film was made. (along with others)A quick google search will show the pictures of them all together. It is widely assumed by cryptos that patterson and gimlin went alone.

Abstract: “A possible function of laryngeal air sacs in apes and gibbons was investigated by examining the relationships between air sac distribution, call rate, call duration and body weight in a phylogenetic context. The results suggest that lack of sacs in the smaller gibbons and in humans is a derived feature.”

Just a brief comment to bring attention to the previous one (many people will miss it, since it was delayed by the spam filter – I’m not sure why). I just want to note my disagreement with your comment about archaic humans being ‘heavy boned’ (and I’m well aware that this description is not unique to you). To my knowledge there is no indication that any fossil hominid is any more heavy-boned that any other large terrestrial mammal, and in fact I’m pretty sure that hominids are less ‘heavy-boned’ that a great many fully terrestrial, even cursorial, tetrapods (giraffe limb bones come to mind, not to mention the limb bones of sauropods). So, the fossil hominids you have in mind are probably not ‘heavy-boned’ and, even if they are, heavy bones are not unique to wading/diving tetrapods.

I see no reason why hominids of some or many species couldn’t have foraged on mudflats, waded etc. But, as I’ve said before, if you’re saying that this is synonymous with the AAH, then you’ve changed the goalposts, since the corner-stone idea of the AAH is that we went through a strongly aquatic phase in our ancestry that involved persistent swimming and diving.

The AAT/H has a variety of forms, all of which (after Hardy’s initial one, anyway) are typically very vague. Hardy and Morgan both argued for a discreet period of semiaquaticism; the variety espoused by later writers, including Verhaegen, usually argue that the time period was basically the whole period of human evolution, and that somehow selection pressure to give us (supposedly) semiaquatic features happened thorugh very little water contact, primarily wading and walking along the shore.

Now it’s true that Verhaegen has also claimed more than this, for instance his famous claim that Neanderthals had “snorkel” noses with their nostrils on the top of the nose (!) and that australopithecines are ancestors of chimps and gorillas (“gracile” australopithecines giving rise to chimps and “robust” australopithecines to gorillas). And, really, a bunch of other really silly things, which interested parties can see if they wish by going over to the page on him and his ideas at my site, Aquatic Ape Theory: Sink or Swim?. And the direct link to the Marc Verhaegen page.

Of all these, australopithecines had the broad pelvis, moderately long legs, and fully vertical gait. At least the robust ones lacked the allegedly reduced mastication (though if I compare my molars to a chimp’s, well…).

I feel obliged to make a few comments:
– The article “Sink or swim” is not worth an answer: it’s outdated, uninformed, misunderstanding & misrepresenting. It didn’t even grasp that when I ironically said that I believe everything until it’s proven wrong, this was about the savanna story…
– The level of misinformation of some anti-AAT people can be seen in their claiming here that I said that gracile australopiths gave rise to chimps, and robust ones to gorillas. Why don’t these people inform before saying something? It’s prof.Kleindienst who first proposed that hypothesis, later repeated (without having read Kleindienst they told me) by Gribbin & Cherfas “The monkey puzzle”. My views can easily be found at google “verhaegen human evolution”: I argued that the E.Afr.apiths afarensis-aethiopicus-boisei & the S.Afr.apiths africanus-robustus evolved in parallel, possibly belonging to the genera Gorilla & Pan resp., the gracile apiths living in swamp forests, the robust ones in more open wetlands, where they, perhaps like lowland gorillas in forest bais, fed on waterside & aquatic herbs such as sedges & papyrus, as well as on hard-shelled invertebrates (hypotheses of Nikolaas van der Merwe & Alan Shabel resp., google “merwe boisei papyrus” & “shabel durophage”).
– Also wrong is Darren’s statement that “there is no indication that any fossil hominid is any more heavy-boned than any other large terrestrial mammal … even if they are, heavy bones are not unique to wading/diving tetrapods.”
Please discern between apiths & Homo: the littoral theory is about Homo, not about apes or apiths. Most erectus fossils have skulls twice as thick (pachyostosis) as gorilla skulls, and almost twice as thick as sapiens (eg, S.G.Gauld 1996 Am.J.phys.Anthrop.100:411-426). Moreover, erectus bones have narrower marrow (medullar steonosis) & denser tissues (osteosclerosis), so most erectus skulls can be estimated to be at least 3 times as heavy as equally large ape skulls. Pachyosteosclerotic skeletons, as every marine biologist knows, are typical of slow shallow divers, eg, S.I.Madar 2007 J.Paleontol.81:176-200: “Bone ballast in the form of osteosclerosis and hyperostosis is an unmistakable hallmark of an aquatic lifestyle” (OTOH, fast swimming Cetacea often have lightly-built skeletons).
Most likely, Pleistocene Homo dispersed between Pakefield, the Cape & Flores along coasts – why wouldn’t these handy, intelligent, tool-using, omnivorous human ancestors not also have dived there for shell- or crayfish on the vast new territories on the continental shelves during glacials. No wonder they got heavier bones. And no wonder we still have thick SC fat tissues, our head-spine-legs in 1 line, a remarkably poor olfaction & small mouth (seafood, see also DHA), etc.
For serious & updated info on AAT please google “econiche Homo”.

It didn’t even grasp that when I ironically said that I believe everything until it’s proven wrong, this was about the savanna story…

Sarcasm is very difficult to get across in writing. You should have known this and should have expressed yourself more clearly.

The first thing I was taught when I started writing scientific papers was: “you will be misunderstood, so try to minimize the opportunities for this to happen”. If you, as a scientist, are misunderstood, it is your fault for not having been even clearer.

OTOH, fast swimming Cetacea often have lightly-built skeletons

And collapsible lungs, so they don’t need to counter the buoyancy of their lungs by having heavier bones and can afford to be lighter and more maneuverable.

Importantly, osteosclerosis (and osteoporosis as seen in cetaceans) can only be diagnosed from mid-shaft cross sections of whole bones. Pachyostosis (which usually appears together with osteosclerosis), however, is visible from the outside: sea cow ribs are banana-shaped.

Why would H. erectus have a pachyostotic skull (something that is very rare — I don’t know of any cases other than Placodus) and osteosclerotic femora, but not pachyostotic ribs?

Most likely, Pleistocene Homo dispersed between Pakefield, the Cape & Flores along coasts – why wouldn’t these handy, intelligent, tool-using, omnivorous human ancestors not also have dived there for shell- or crayfish on the vast new territories on the continental shelves during glacials. No wonder they got heavier bones.

Then why are they also found in places far away from coasts, like southern Germany or central northeastern China? Pachyosteosclerotic bones are a disadvantage for walking, which is why terrestrial animals are never pachyostotic and why only the smallest ones are osteosclerotic.

Sarcasm can indeed be difficult to get across in writing, but the context of what Verhaegen said shows that he is lying when he says his “I believe everything what I read” remark was about savannas. It was about the source of his information on Neaderthal nostrils (and about a piece of writing on someone who one should definitely not believe, even if one was inclined to believe, as Verhaegen did, that Neanderthals had “snorkel noses” with their nostrils on the top). (That info, about Marc’s several-year-long attempts to rewrite the history of his embarrassing quote, is on my site, BTW.)

Marc’s claim about australopithecine ancestral to gorillas and chimps is seen in his online statements like this one from 2005: “I now think (AFAIK I’m the only one :-D) that aethiopicus-boisei belonged to Gorilla, and africanus-robustus to Pan, both lineages evolving in parallel (gracile>robust) when climates became cooler & drier c 2.2 Ma or so.” I don’t care, or mention, who came up with the idea first; Marc has been pushing it for years… since the mid-1990s at least.

If you read the page on Marc at my site, you’ll find that these are actually among the least problematic of his many odd claims.

The photo shows that DDeden is indeed correct here.
Feeding on floating vegetation (what this gorilla is doing & what most or all australopiths at least parttime did) is the opposite of slow diving for sessile littoral foods (what pachyostotic animals (including H.erectus) do, as all marine biologists know).
Contrary to what Darren seems to believe, airsacs are no evidence for AAT, to the contrary: airsacs hinder diving, of course.
H.erectus & relatives did not disperse to different continents & islands running over savannas as some PAs still believe (see fantasies like endurance running & Savannahstan), but along coasts & rivers: cursorial animals are lightly-built, and there are no savannas to Flores.
See my recent paper with Stephen Munro in HOMO J.compar.hum.Biol.62:237-247 “Pachyosteosclerosis suggests archaic Homo frequently collected sessile littoral foods” and for recent information on AAT see our forthcoming book Mario Vaneechoutte, Algis Kuliukas & Marc Verhaegen eds “Was Man More Aquatic in the Past? Fifty Years after Alister Hardy: Waterside Hypotheses of Human Evolution”, with contributions of Elaine Morgan, prof.Tobias etc.
People who want to discuss AAT should at least be informed.

Indeed. They walk and run on their toes/nails. We run on the balls of the feet, and walk on the flat soles. Australian lifeguards found that the most efficient way to move on beach sand was to walk on flat soles, to avoid inefficient slipping. Large animals moving on loose flat slippery ground tend to have broad feet eg. caribou.
Savanna ungulates instead have sharper less broad feet, for better high-speed traction and continuous walking on firm ground.

The last common ancestor of Gorilla, Pan & Homo et al was neither a habitual swimmer nor habitual ground walker, but was primarily a waterside arboreal biped with small inflatable laryngeal air sacs which developed with increasing size and reliance on below-branch action, the air sac naturally selected as an air cushion in high falls into water (buoyancy) and ground (rib-lung cushion). (Sloths, gibbons and chimps notoriously get broken long bones from falls of great heights, they are adept at climbing with 3 limbs.) The further size increase of air sacs in mature male gorillas and orangs is a result of this advantage, despite being less arboreal than females, they still climb trees during fruiting. Humans seldom do. Vocalizing, upper body enlargement are results, rather than causes for air sacs in hominoids. Small gibbons and sloths are too small for air sacs to be air cushions. Slow vertical divers don’t need air sacs.

*sigh* The fact that we walk plantigradely doesn’t require any special explanation. It’s simply retained. All primates are plantigrade. All mammals, except for derived exceptions, are plantigrade. All limbed vertebrates, except for derived exceptions, are plantigrade!

“Everything is the way it is because it got that way.” — J. B. S. Haldane

All we can tell is that we haven’t been under selection for digitigrady, at least not for long.